Paladin,
You have received some good advice. However in reading what you have said, there is a major problem, in my opinion. First, you are not experienced in range/shooting safety issues, according to your post. Second, your family member or bro-in-law is an unknown entity to you. This, to me is a recipe for disaster. Is he a safe person? Does he know gun safety? Just because he hunts, doesn't mean he knows anything. I have hunted with people who I will never be around again due to very unsafe gun handling--and this includes one guy who is a NRA certified instructor. I have had my children shooting since age 4. If anyone joined us while shooting, it was only close friends who I had spent time shooting or hunting with and were very safe. It is not always possible to have a second chance. And, no matter how safe people are, accidents can happen. Hence, we must limit our liability.
Third, you have mentioned your sons proclivity. Based on your statement, as has been suggested, I would get a BB gun ( and some type of reactive target)and work one on one with him. This means bonding and learning time together without others and without gunfire to distract you or your son. Even with the proper protection gear, having certain rifles or pistols discharging in the same general vicinity, can cause one to jump, flinch and just feel unconfortable. Stay away from guns with comps. This may have a long term negative affect on your son.
Therefore, I would have a descreet conversation with your bro in law and only you can judge his demeanor. If you go, then devote yourself to being with your son and not to you shooting.
My suggestion is for you to get some experience and training first, so you can feel comfortable around guns and safety issues. Then introduce your son to shooting as stated and then move to rimfire's.
Good luck, Steve